Optical exposure systems are the current technology of choice for patterning photosensitive polymers in manufacturing applications. However, in the sub-micron world that looms on the horizon, exotic photoresist compositions and complex processing techniques will become increasingly necessary in order to prolong the viability of optical exposure systems. Accordingly, alternate photoresist exposure systems are being explored in the hope that they will fulfill the stringent manufacturing requirements of tomorrow's technology.
One particularly promising technology is electron beam (E-beam) exposure. In these systems, beams of electrons are irradiated on a surface to be patterned. In a particular application referred to as "direct-write E-beam exposure," these electron beams are controlled by an imposed electric field to expose selected areas of a photoresist layer, rather than exposing selected areas of the photoresist through a metallic mask as in conventional optical exposure systems. Since these metallic masks are costly to design and produce, the combined advantages of printing images at tighter geometries and eliminating metallic masks make direct-write E-beam systems very attractive.
An article by Kenty et al, entitled "Electron Beam Fabrication of High Resolution Masks," J. Vac. Sci. Tech., October-December 1983, pp. 1211-1214 discusses a particular patterning method for E-beam exposed resists. A quartz plate coated with polymethyl methacrylate (PMMA) was exposed to a direct-write electron beam at 20 kev in order to form 0.5 m features upon wet development. The resist pattern was then ion implanted with silicon, and the resulting mask was found to work well as a photomask for optically exposing other photoresist materials. See also an article by Maclver, entitled "High Resolution Photomasks with Ion-Bombarded Polymethyl Methacrylate Masking Medium," J. Electrochem Soc.: Solid-State Sci. & Tech., April 1982 pp. 827-830.
In an article by Iida et al, entitled "An Approach to Quarter-Micron E-Beam Lithography Using Optimized Double Layer Resist Process," IEDM Digest of Technical Papers 1983, Paper 25.6, pp, 562-565, an E-beam is used in a direct write mode to pattern an upper thin layer of photoresist. The patterned photoresist is in turn used to pattern an underlaying thicker polyimide layer in an O.sub.2 plasma. As shown in FIG. 2 of the paper, by patterning only the upper 0.4 .mu.m photoresist layer be direct-write E-beam techniques, a 20 kev acceleration voltage produces better results than patterning a single, 1.8 .mu.m photoresist layer at a 120 kev acceleration voltage.
In an article by Ishii et al, entitled "A New Electron Beam Patterning Technology for 0.2 .mu.m VLSI," 1985 VLSI Symposium, May 14-16 1985, Kobe, Japan, paper VIII-1, pp. 70-71, a direct-write E-beam system (acceleration voltage of 30 kev) is used to partially expose and pattern a photoresist layer. That is, only the upper portion of the photoresist layer is patterned. Then a second layer is deposited on the photoresist layer. The second layer is etched back so that portions of the second layer remain in the pattern that was formed in the photoresist. Finally, the photoresist is etched under conditions that do not etch the remaining portions of the second layer, so that the exposed portions of the photoresist are removed. A silicon resin was used as the second layer and PMMA was used as the photoresist. Note that in order to adequately define the final pattern, the etchback of the second layer had to be continued until at least half of the patterned portion of the photoresist layer was removed.
Both of the above Iida and Ishii articles are directed to the same general idea. In order to completely pattern single photoresist layers of a thickness of 1 .mu.m or greater, the acceleration voltage must be kept at a high level and the photoresist must be exposed to the E-beam for a longer period of time (in other words, the E-beam "dose" necessary to pattern a conventional photoresist layer is high). As the dose increases, the throughput of the exposure tool decreases. Thus, others such as Iida and Ishii have attempted to decrease the necessary dose (to decrease dwell time and hence increase throughput) by totally patterning a thin layer (Iida) or by partially patterning a thick layer (Ishii).
Accordingly, there is a need to formulate other processes that increase the throughput of direct-write E-beam exposure systems.